A voltage-controlled oscillator (“VCO”) is an important functional block in many types of analog, digital, and mixed-signal circuits. For example, VCOs are used in phase-locked loops (“PLLs”) that are used in clock generation, tuning circuits, and frequency synthesis. There are many types of VCOs. A type of VCO that is often used in PLLs on integrated circuits (“ICs”) is a ring oscillator.
A ring oscillator generally passes a signal pulse (such as an input clock signal) from one delay element to the next. The amount of delay each delay element provides is typically controlled by selecting the bias current or bias voltage applied to the delay elements. Typically, a higher bias current decreases the delay of the delay element(s), and the frequency of the ring oscillator increases. Even though some ring oscillators operate according to the amount of current provided to them, these ring oscillators are commonly referred to as “VCOs”, which is used below to describe current-controlled ring oscillators for convenience of discussion.
However, the performance and operating point of the PLL can depend on many factors, such as variations in the fabrication process, operating temperature drift, and supply voltage. Many PLL systems have a mechanism for adjusting the operating point of the PLL. One technique is to open the PLL and set the frequency of the VCO with an analog circuit to a desired center frequency. The PLL is then closed, and the PLL circuitry locks the VCO to a reference. However, opening the PLL removes some components of the PLL that might otherwise contribute to setting the center frequency; hence, the effect these elements have on the nominal operating frequency is not accounted for, and may pull the VCO frequency above or below the desired center frequency when the PLL is closed.
Similarly, although opening the PLL and adjusting the center frequency of the VCO improves the operation of the VCO at power-up, factors that affect the operation of the VCO can change over time. For example, local changes in the power supply voltage and operating temperature, which can be caused by external effects, cause the nominal operating point of the VCO to drift. Such drift creates an asymmetry in the PLL to adjust for over- and under-frequency conditions. For example, if the nominal operating point of the VCO increases with increasing temperature, some portion of the dynamic tuning range of the PLL is used to pull the VCO back down to the desired center frequency, limiting the range available to pull the VCO down to maintain phase lock.
Another technique uses fast and slow analog feedback loops to control a VCO. The fast analog feedback loop is used to track the reference signal in the PLL, and the slow feedback loop is used to maintain the operating point of the VCO near the desired center frequency according to a differential voltage going into the VCO transconductance circuit. The response time of the slow loop is sufficiently long to avoid interfering with phase locking (“loop dynamics”) of the PLL. In some cases, physically large components are used in the slow analog loop to slow the response time, and the gain of the loop amplifier can be difficult to adjust. Similarly, process variations and temperature variations can affect the performance of the slow analog feedback loop, both initially and during operation of the PLL.
Therefore, there is a need for improved PLLs.